[reference][validation] Adding a quick doc for the UniqueEntity constraint

Author: weaverryan

reference/constraints.rst

@@ -21,6 +21,7 @@ Validation Constraints Reference
    constraints/MinLength
    constraints/NotBlank
    constraints/NotNull
+   constraints/UniqueEntity
    constraints/Regex
    constraints/Time
    constraints/Url
@@ -52,6 +53,7 @@ The following constraints are natively available in Symfony2:
 * :doc:`MinLength <constraints/MinLength>`
 * :doc:`NotBlank <constraints/NotBlank>`
 * :doc:`NotNull <constraints/NotNull>`
+* :doc:`UniqueEntity <constraints/UniqueEntity>`
 * :doc:`Regex <constraints/Regex>`
 * :doc:`Time <constraints/Time>`
 * :doc:`Url <constraints/Url>`

reference/constraints/UniqueEntity.rst

@@ -0,0 +1,46 @@
+UniqueEntity
+============
+
+Validates that a particular field (or fields) in a Doctrine entity are unique.
+For example, suppose you have an ``AcmeUserBundle`` with a ``User`` entity
+that has an ``email`` field. You can use the ``Unique`` constraint to guarantee
+that the ``email`` field remains unique between all of the constrains in your
+user table:
+
+.. configuration-block::
+
+    .. code-block:: php-annotations
+
+        // Acme/UserBundle/Entity/User.php
+        use Symfony\Component\Validator\Constraints as Assert;
+        use Symfony\Bridge\Doctrine\Validator\Constraints as DoctrineAssert;
+        use Doctrine\ORM\Mapping as ORM;
+
+        /**
+         * @ORM\Entity
+         * @DoctrineAssert\UniqueEntity("email")
+         */
+        class Author
+        {
+            /**
+             * @var string $email
+             *
+             * @ORM\Column(name="email", type="string", length=255, unique=true)
+             * @Assert\Email()
+             */
+            protected $email;
+            
+            // ...
+        }
+
+    .. code-block:: yaml
+
+        # src/Acme/UserBundle/Resources/config/validation.yml
+        constraints:
+            - UniqueEntity: email
+
+Options
+-------
+
+* ``fields``: The field (or list of fields) on which this entity should be
+  unique. For example,